1. Field of the Invention
The present invention is a method for the simultaneous, independent measurement of the moisture content and bulk density of any hygroscopic particulate material, such as, for example, agricultural products such as grains, from radio-frequency measurements of the dielectric properties of the bulk material.
2. Description of the Related Art
Moisture content of materials is a key parameter in many research and industrial applications, including the food and agriculture-related industries. The most widely used standard techniques for moisture content determination are oven drying techniques. These techniques are based on drying samples under specific conditions, such as temperature and time, depending on the material. Besides being energy and time consuming, in some instances the representative character of the samples might be questionable compared to the whole volume or mass of material under consideration. Moreover, most industrial processes are highly automated and require real-time, on-line measurement of the moisture content.
Electromagnetic wave-interaction-based techniques meet this requirement and provide a tool for continuous measurement. In this way, by averaging, a better estimate of the moisture content can be achieved. Among these techniques, free-space microwave techniques have the advantages of being nondestructive and contactless. Therefore, they are suitable for on-line, real-time monitoring and control. However, with particulate materials, bulk density fluctuations, as material moves on a conveyor belt or flows through a pipe, can produce significant errors in moisture content determination. It is possible to reduce these fluctuations by mechanical means by keeping the layer thickness constant or by using a vibrator to maintain an average density. However, this still produces unpredictable errors in moisture content because of the density effect. In this instance, the density has to be determined by a separate method, such as gamma-ray attenuation or weighing. A separate density measurement is always an additional cost with more technical complications in the design and implementation of the measuring system.
A better alternative is to identify empirically or define theoretically density-independent functions exclusively dependent on moisture content. From an industrial perspective, the concept of density independence is a convenient solution for a cost-effective meter that fulfills specific requirements. Therefore, the density-independent functions should be easy to manipulate for moisture content computation and tolerate instabilities produced by the measuring system and the immediate environment as well. Most of the transmission systems for moisture content determination are based on the principle of two-parameter measurement, namely the attenuation .DELTA.A and phase shift .DELTA..phi. and use of the ratio ##EQU1## as a density-independent function (Kraszewski et al., J. Microwave Power, Volume 12 (3), 241-252, 1977). This ratio was identified empirically and can be used only in a transmission configuration over a limited moisture content range (Menke et al., IEEE MTT-S International Microwave Symposium Digest, Volume 3, 1415-1418, 1996).
To generalize the concept of density independence, the function has to be expressed in terms of universal entities such as the dielectric properties. The dielectric properties of materials are intrinsic properties usually expressed by the relative complex permittivity, .epsilon.=.epsilon.'-j.epsilon.", where .epsilon.' is the dielectric constant, which represents the ability of a material to store electric energy, and .epsilon." is the loss factor, which represents the loss of electric-field energy in the material. Another parameter often used to describe the amount of loss is the loss tangent, tan .delta., defined as the ratio ##EQU2## The dielectric constant and loss factor, as well as the loss tangent, of moist substances are generally dependent on frequency, temperature, density, and moisture content. The influence of these variables on the relative complex permittivity has been explored and reported for many materials (Nelson et al., J. Agric. Eng. Res., Volume 21, 181-192, 1976; Kent, J. Microwave Power, Volume 12 (4), 341-345, 1977; Meyer et al., IEEE Trans. Microwave Theory Techn., Volume MTT-29 (7), 732-739, 1981; Nelson, Cereal Chemistry, Volume 58 (6), 487-492, 1981; Nelson, J. Microwave Power, Volume 18 (2), 143-153, 1983; Kress-Rogers et al., J. Food Eng., Volume 6, 345-376, 1987; Kraszewski et al., J. Microwave Power and Electromagn. Energy, Volume 31 (3), 135-141, 1996).
Present state-of-the-art microwave moisture measurement systems attempt to eliminate density fluctuation effects by secondary measurements of density with gamma radiation gauges or other techniques, or by taking the ratio of attenuation and phase-shift in microwave measurements. These techniques limit the errors in moisture content determination attributable to fluctuations in bulk density, but seldom do they eliminate the density effects entirely. Also, secondary measurements of density complicate measurement systems and increase their consequent costs.
Bussey (Proc. IEEE, Volume 55 (6), 1046-1053, June 1967) discussed the use of microwave resonant cavity techniques to measure the microwave and dielectric properties of uniformly-shaped materials by measuring the shift in the resonant frequency and the change in the Q-factor for the cavity when the sample is inserted into the cavity.
A resonant cavity has been applied for determining moisture content in uniformly shaped single seeds by simultaneous measurements of resonant frequency shift and the transmission factor (Kraszewski et al., IEEE Trans. Instrum. Meas., Volume 38 (1), 79-84, 1989; J. Agric. Eng. Res., Volume 48, 77-87, 1991; U.S. Pat. No. 5,039,947 ('947), 1991). Kraszewski et al., 1989, disclose a nondestructive process for the determination of moisture content in single soybeans using a microwave resonator. A seed is placed in a microwave resonant cavity and the resonant frequency shift and change in Q-factor are measured. This process allows the measurement of moisture content of particles of nearly uniform spherical shape. Kraszewski et al., 1991 and '947 disclose a nondestructive process for determining the moisture content of particles of irregular or variable shape where the irregular or variable-shaped product is inserted into a microwave resonant cavity in a first position and the energy dissipated in the product and the shift or change in the resonant frequency (or wavelength) due to the presence of the product is measured. The orientation of the product is then changed to a second position which is rotated by n.times.90 degrees with respect to the maximum field vector (n is an odd integer) and the measurements are repeated.
Kraszewski et al., (Trans. ASAE, Volume 36(1), 127-134, 1993) disclose a method for the simultaneous measurement of moisture content and mass in single peanut kernels, which are also of nearly uniform shape, using microwave resonator measurements of resonant frequency and change in cavity transmission characteristics. The cavity consisted of a section of standard WR-284 rectangular waveguide (inside dimensions: 72.times.34 mm) 305 mm long operating in the H.sub.105 (TE.sub.105) mode. It was coupled with external waveguides through two identical coupling holes 20.6 mm in diameter at each end of the cavity. A PLEXIGLAS.TM. tube of 15.8 mm outside diameter and 12.4 mm inside diameter was installed in the center of the cavity which supports the peanut kernel at the center of the cavity.
While various methods have been developed for measurement of properties of different materials, there remains a need in the art for a method for simultaneous, independent real-time measurements of bulk density and moisture content of hygroscopic particulate materials. The present invention provides a method which is different from prior art methods and solves some of the problems associated with the measurement of density and moisture content of bulk materials.